Beijing has dismissed Tibetans, young and old, who have set themselves on fire in protest against Chinese rule as criminals or suspicious people with a "very bad reputation".

Despite extensive security, the government has failed to stop self-immolations, which have attracted attention and sympathy across the world. More than 20 monks, nuns, former clergy and ordinary people – including three in the last five days – have set themselves on fire in the past year, mostly in Aba county in south-western Sichuan province.

"Some of the suicides are committed by clerics returning to lay life, and they all have criminal records or suspicious activities. They have a very bad reputation in society," said Wu Zegang, China's top official in Aba.

Authorities have sealed off Aba, making it impossible to verify details of the cases. Officials have suggested the acts were linked to personal issues, most recently claiming that a 20-year-old woman who set fire to herself in Gansu province on Saturday had lost "her courage for life and study" when her school performance suffered, after she hit her head on a radiator.

Wu said those who had set themselves on fire in Aba had shouted pro-independence slogans beforehand. He cited it as evidence that the self-immolations were "orchestrated and supported" by Dalai Lama and Tibetan independence forces.

The Dalai Lama, who lives in exile in India, has praised the courage of those who engage in self-immolation but has said he does not encourage the protests. He blames Chinese repression of Tibetan culture and religion for the cases.

"I think China miscalculated in the late 90s, by exporting to eastern Tibetan areas aggressive anti-Dalai Lama policies they had been imposing for years in central Tibetan areas," said Professor Robert Barnett, an expert on Tibet at Columbia University. "Since people began to protest the policy has been more hardline."

The recent outbreakspate of self-immolations began with the death of Phuntsog, a young monk from Kirti monastery, last March. The response of authorities — surrounding the monastery and later sentencing monks including his brother and uncle for involvement with his death — appeared to exacerbate tensions.

Stephanie Brigden of Free Tibet said it was ludicrous to describe those who had self-immolated as criminals. She said it reflected the officials' anxiety over failure to stop the burnings.

"In the last few months we have seen lots of efforts by China to criminalise them and even use the language of terrorism. I think it's a result of the pressure China feels internally as well as externally.

"Local leaders have been told to maintain social stability; they know it is not within their capacity so their only strategy is to use rhetoric that criminalises people. They have responded with force and it's not worked."

Barnett says officials appear to be in an intellectual quandary with their Tibet policy, responding to complaints of excessive state interference with more interference.

The increasing reach of officials into monastery life is underlined by a new "six ones" policy for officials in new temple management committees being set up in the Tibet autonomous region. The six instructions include becoming close friends with at least one monk or nun and handling one problem so their families feel the warmth of the party and the government – but, at the same time, also building one file on each of them.

"China's strategy over the last 30 years of winning Tibetans over through generosity is running so closely to its other strategy of intimidating people who do not have the right views," said Barnett.

Meanwhile, leaders from north-western Xinjiang region, which has experienced ethnic violence, vowed to attack what the Communist party secretary Zhang Chunxian called "rotten eggs and bad elements" in a briefing on Wednesday.

"When an event occurs, we resolutely smash it. When an incident occurs, we also smash it," he had said.